1. Field of the Invention
The present invention relates to a spread spectrum communications system and receiver capable of demodulating received data without regenerating a carrier.
2. Description of the Related Art
The spread spectrum communications are established using a signal spread into a band much larger than a required frequency band in transmitting data. This spread spectrum communications system can be a direct sequence (DS) system, a frequency hopping system, a time hopping system, etc. depending on the spreading method. Described below is the DS system.
In the DS system, the transmission data is spread and modulated by multiplying the transmission data by a pseudo-noise (PN) code (spreading code) in a transmitting equipment to obtain a broadband spectrum spread signal. A PN code is used to generate binary data having a much higher frequency than normal transmission data. The width of the spectrum of the transmission data spread using the PN code equals the bandwidth of the PN code. The above described modulation is normally accompanied by the PSK modulation.
In the receiving equipment, a reverse-spreading process is performed by multiplying the data modulated in the transmitting equipment by the PN code used by the transmitting equipment so that the original transmission data can be retrieved. At this time, the phase of the PN code by which the received data is multiplied should be the same as that of the PN code used by the transmitting equipment. Therefore, a synchronous process is performed for phasing.
FIG. 1 is a block diagram showing the synchronous detection circuit in a receiving equipment of the above described spread spectrum communications system. An RF/IF circuit 101 converts the data transferred on the carrier of an RF band into the data of an IF band. A multiplier 102a multiplies the output data from the RF/IF circuit 101 by the carrier regenerated by the unit described later. The data output from the multiplier 102a passes through a low pass filter 103a and is input to a correlation circuit 104a. The correlation circuit 104a multiplies the input data by the PN code generated by a PN code generation circuit 105a, and outputs the timing-correlated data as regenerated data. The units 102b through 105b perform operations similar to those of the units 102a through 105a respectively.
A multiplier 106 multiplies the output of correlation circuits 104a and 104b (or low-pass filters 103aand 103b). A carrier regeneration circuit 107 comprises a voltage-controlled oscillator (VCO), and outputs a sine wave (regenerated carrier) at the frequency corresponding to the output voltage of the multiplier 106. The regenerated carrier output by the carrier regeneration circuit 107 is directly input to the multiplier 102b, and input to the multiplier 102a through a .pi./2 phase difference circuit 108.
Thus, in the synchronous detection circuit, a carrier is regenerated from received data, a received signal is converted into a base band using the regenerated carrier, the PN code is phase-synchronized between the transmitting equipment and the receiving equipment, and the data is retrieved in the receiving equipment.
The delay detection circuit shown in FIG. 2 is known as another configuration from which data is retrieved in the receiving equipment of the above described spread spectrum communications system. A multiplier 111 multiplies the data transferred on the carrier by the output from a delay circuit 112. The delay circuit 112 delays the data transferred on the carrier by a predetermined value (equal to the delay value used in differential coding in the transmitting equipment, for example, one chip of the PN code). The output of the multiplier 111 passes through a low-pass filter 113 and is input to a correlation circuit 114. The correlation circuit 114 multiplies the input data by the PN code generated by a PN code generation circuit 115, and outputs the timing-correlated data as regenerated data.
The synchronous detection circuit shown in FIG. 1 regenerates a carrier from received data using the carrier regeneration circuit 107. Since most carrier regeneration circuits 107 contain VCO and so on and perform analog processes, there is a drawback such that the entire circuits are large in scale.
The carrier should be precisely regenerated by the carrier regeneration circuit 107 so that the frequency of the regenerated carrier can match the frequency of the actual carrier (in this example, the frequency of the carrier converted by the RF/IF circuit 101). That is, when the frequency of the actual carrier is different from the frequency of the regenerated carrier, the correlation circuits 104a and 104b cannot detect a precise correlation timing, thereby failing in correctly regenerating data. However, as the frequency of a carrier becomes high, it is more difficult to regenerate the carrier and the entire cost increases because the electronic parts for high precision are normally expensive.
The delay detection circuit shown in FIG. 2 has also the disadvantage that the circuit scale is very large as with the above described synchronous detection circuit because most of the processes of the circuit are performed in an analog format.
The delay value of the delay circuit 112 is represented by, for example, one chip of the PN code, and should be very precise. That is, when a delay value error becomes large, a correct correlation timing cannot be detected by the correlation circuit 114, thereby failing in regenerating data. However, the required precision of a delay value is nanosecond in order. But, it depends on the band of the PN code, and it is hard to adjust the precision. Besides, there is the problem that the entire cost increases because the electronic parts for high precision are normally expensive.